Brucella suis, Brucella abortus, and Brucella melitensis were shown by microscopic and cultural procedures to multiply extensively within normal rat, mouse, and guinea pig monocytes maintained in vitro in cell cultures for 3 days. Intracellular growth of brucellae had no observable toxic effects on most monocytes, although many of the cells became completely engorged with brucellae within 3 days. Non-smooth brucellae and strain 19 multiplied slowly within normal monocytes. In contrast, "immune" monocytes) i.e. those derived from animals previously infected with smooth brucellae, greatly restricted the intracellular growth of smooth and non-smooth brucellae and strain 19. Growth of smooth Brucella, within either normal or "immune" monocytes, was not influenced by addition of Brucella antiserum to the culture medium.

Desensitization of immunized guinea pigs did not diminish the refractory state of their monocytes. Cellular resistance did not develop when animals were vaccinated with heat-killed brucellae, though these animals did produce agglutinating antibody. Similarly, vaccination of animals with living, rough B. suis failed to induce a refractory state in their monocytes, even though the vaccinated animals developed delayed hypersensitivity to smooth Brucella antigen.

In vivo studies of Brucella survival in the spleens of normal and vaccinated mice (treated with streptomycin to prevent extracellular survival) gave strong support to the in vitro demonstrations of acquired "cellular immunity." Some implications of these results are discussed.

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